This invention relates to ringing signal generators for telephone systems and, more particularly, to a ringing signal generator for a subscriber loop carrier system which generates the high-level ringing signal by amplifying a low-level reference ringing signal.
It has become increasingly common to utilize carrier techniques to establish communication paths between a telephone central office and a plurality of remotely located telephone subscribers. Where such systems were, at one time, economically justifiable only for extremely long rural routes, today, due to the rapidly decreasing costs of integrated micro-electronics, such systems are becoming economically feasible for shorter and shorter subscriber loops.
In a subscriber loop carrier system, a plurality of voice channels are derived on one or two pairs of wire conductors by using analog carrier frequency division techniques, or by using time division digital techniques. In either case, the metallic connection previously used for transmitting subscriber ringing signals to the subscriber location can no longer be used for this purpose. It has, therefore, become necessary to generate ringing signals at the remote terminal of the carrier system and to control the generation and application of these ringing signals by supervisory information transmitted over the carrier-derived channels.
Since remote ringing signal generators serve a relatively small number of subscribers and are housed at the remote location, it is desirable that such ringing signal generators be small, inexpensive, compact, and require little power. The latter desirable characteristic is necessary to minimize battery back-up requirements. One way to generate high-level ringing signals is to linearly amplify at the remote terminal a low-level reference ringing signal, which has the desired signal shape and frequency characteristics of the ringing signal to be applied to the subscriber loop. Standard linear amplification techniques, however, are not power-efficient. Therefore, standard amplifier circuitry does not meet the aforenoted requirements for remote terminal equipment.